Recently, a noise control device has been proposed in which control (so-called active noise control) to reduce a noise arriving at a control point by reproducing a control sound which is in antiphase to the noise arriving at the control point is realized through selective operation of fixed filters and adaptive filters (for example, Patent Document 1).
Hereinafter, with reference to FIG. 27, the conventional noise control device mentioned above will be specifically described. FIG. 27 is a diagram illustrating a circuit structure of a conventional noise control device. In FIG. 27, the noise control device includes a noise microphone 9101, an adaptive filter 9201, a control speaker 9401, an error microphone 9501, and a fixed filter 9601.
The noise control device shown in FIG. 27 selects the adaptive filter 9201 to control a noise when the noise has changed due to a change of the position of the noise source and/or the generating state of the noise (for example, driving condition of a fan: the number of rotations). The noise microphone 9101 detects the noise arriving from the noise source, to output a noise signal to the adaptive filter 9201. The adaptive filter 9201 signal-processes the inputted noise signal by using a filter coefficient, to generate a control signal. The control signal generated at the adaptive filter 9201 is emitted as a control sound from the control speaker 9401 toward the control point. The error microphone 9501 is provided at the control point, and detects the noise arriving at the control point from the noise source and the control sound arriving at the control point from the control speaker 9401. At the error microphone 9501, the noise arriving at the control point interferes with the control sound from the control speaker 9401, and a difference therebetween is detected as an error signal. The adaptive filter 9201 has its filter coefficient updated to minimize the error signal. A specific method for this is, for example, the Filtered-X_LMS algorithm. Hereinafter, the update processing using the Filtered-X_LMS algorithm or the like will be referred to as coefficient update processing. In the coefficient update processing as mentioned above, when the noise has changed due to a change, for example, of the position of the noise source and/or the generating state of the noise, the adaptive filter 9201 is able to have its filter coefficient updated so as to generate an optimum control signal in accordance with the changed noise arriving at the control point.
Once the filter coefficients updated at the adaptive filter 9201 have converged, the noise control device shown in FIG. 27 selects the fixed filter 9601 having the coefficient value at which the filter coefficients have converged, as a fixed coefficient, thereby performing noise control.
Thus, the noise control device shown in FIG. 27 selectively operates the fixed filter and the adaptive filter to realize the active noise control.    PATENT DOCUMENT 1: Japanese Laid-open Patent Publication No. 2-285799